1. Field of the Invention
The present invention relates to a radio frequency amplifier circuit for amplifying a radio frequency signal used by a transmission section of a mobile communication terminal such as a mobile telephone, and a mobile communication terminal using the radio frequency amplifier circuit, and more particularly, to a technique of controlling a bias current (output power) of the radio frequency amplifier circuit by using a control signal.
2. Description of the Background Art
Recently, in a mobile communication field, not only sound communication function but also data communication function of distributing an image and music has been developed. Therefore, a mobile communication terminal having an enhanced data communication function becomes predominant. For example, as the W-CDMA (Wideband Code Division Multiple Access) system, the HSDPA (High Speed Downlink Packet Access) system and the HSUPA (High Speed Uplink Packet Access) system have been developed so as to increasingly enhance a speed at which data are transmitted between a base station and the mobile communication terminal. Further, a multimode communication system in which the sound communication is performed by using the conventional W-CDMA system (Release99), and a high speed data communication is performed by using the HSDPA system and the HSUPA system, has been developed.
For example, the multimode mobile communication terminal as described above requires a radio frequency amplifier circuit to represent an enhanced linear characteristic in the HSDPA system used for the high speed data communication as compared to in the conventional Release99 used for the sound communication. In general, it is necessary to increase an operating current of the radio frequency amplifier circuit representing the enhanced linear characteristic. Therefore, the mobile communication terminal applicable to multiple communication systems is required to include a radio frequency amplifier circuit designed for the HSDPA system used for the high speed data communication so as to represent the enhanced linear characteristic. Therefore, consumption of current is increased in a normal mode used for the sound communication which is more frequently used than the high speed data communication. Further, diversified applications of the mobile communication terminal complicates a processing circuit, thereby increasing the consumption of current. Therefore, it is important to reduce the consumption of current in the radio frequency circuit block.
Hereinafter, a conventional mobile telephone terminal applicable to multiple communication systems used in the W-CDMA system will be described.
FIG. 23 is a block diagram illustrating a configuration of a radio communication section of the conventional mobile telephone terminal. As shown in FIG. 23, the radio communication section of the conventional mobile telephone terminal includes: a transmission section 200; a reception section 400; a synthesizer section 300; and a common use unit 500. The transmission section 200 includes: a modulator 201; a radio frequency amplifier circuit 202; a band-pass filter 203; a high power radio frequency amplifier circuit 204; and an isolator 205. The common use unit 500 includes an antenna 501 and a duplexer 502. The reception section 400 includes a radio frequency amplifier circuit 401, band-pass filters 402 and 404, and a demodulator 403. The synthesizer section 300 includes a temperature-controlled crystal oscillator (TCXO) 301, a phase-locked loop (PLL) circuit 302, and a voltage-controlled oscillator (VCO) 303.
The modulator 201 converts an inputted modulation signal into a transmission signal of a transmission frequency (around 1.9 GHz in the case of the W-CDMA system) by using a signal outputted by the synthesizer section 300. The radio frequency amplifier circuit 202 amplifies an output signal of the modulator 201 by changing a gain such that the output signal of the amplifier 201 changes from 1 mW or less up to a maximum of about 10 mW. The band-pass filter 203 extracts a signal of a transmission band from a radio frequency signal having been amplified by the radio frequency amplifier circuit 202. The high power radio frequency amplifier circuit 204 amplifies, by using a fixed gain, the radio frequency signal outputted by the band-pass filter 203 such that the radio frequency signal changes from 10 mW or less up to a maximum of about 1 W. The isolator 205 unidirectionally supplies an output signal of the high power radio frequency amplifier circuit 204 to the common use unit 500.
The duplexer 502 includes a TX terminal connected to an output terminal of the isolator 205, an RX terminal connected to an input terminal of the reception section 400, and an ANT terminal connected to the antenna 501. The radio frequency amplifier circuit 401 amplifies a signal received by the antenna 501 of the common use unit 500. The band-pass filter 402 extracts a signal of a transmission band from an output signal of the radio frequency amplifier circuit 401. The demodulator 403 mixes the signal extracted by the band-pass filter 402 and a local signal supplied by the synthesizer section 300. The band-pass filter 404 extracts an intermediate frequency signal from an output signal of the demodulator 403. The synthesizer section 300 supplies, to the transmission section 200 and the reception section 400, a signal of a predetermined frequency.
Next, the high power radio frequency amplifier circuit for use in a radio frequency circuit block of the mobile communication terminal will be described.
In recent years, the high power radio frequency amplifier circuit for use in a radio frequency circuit block of the mobile communication terminal uses a heterojunction bipolar transistor (HBT) instead of an field effect transistor (FET). Unlike the depression type FET, the HBT does not require a negative voltage for a gate bias, and is capable of performing amplification using only power supply of a positive voltage, thereby simplifying a peripheral circuit. However, the HBT is disadvantageous in that the HBT requires a bias circuit for compensating temperature dependency and power supply voltage dependency. Accordingly, it is important to design the bias circuit so as to represent stable characteristic.
Hereinafter, the conventional radio frequency amplifier circuit will be described with reference to drawings.
For example, an amplifier 100 as shown in FIG. 24 typifies the conventional radio frequency amplifier circuit. See, for example, Japanese Laid-Open Patent Publication No. 2004-40500 (page 7, FIG. 1). In FIG. 24, the amplifier 100 includes a bias circuit 102, a reference voltage supply section 103, and a transistor Q101. The bias circuit 102 includes: a resistance R102; a resistance R103; a transistor Q102; a transistor Q103; and a transistor Q104. The reference voltage supply section 103 includes a resistance R101.
To the resistance R102, a switchover voltage Vmod for switching a set bias is applied at one terminal thereof, and a collector and a base of the transistor Q102 are connected at the other terminal thereof. An emitter of the transistor Q102 is connected to a collector and a base of the transistor Q103. An emitter of the transistor Q103 is grounded. A power supply voltage Vdc is applied to a collector of the transistor Q104, and an emitter of the transistor Q104 is grounded via the resistance R103 and receives a reference voltage Vref applied thereto via the resistance R101. The transistor Q104 receives, at a base thereof, a voltage from the other terminal of the resistance R102, and outputs the voltage from its emitter. The output from the emitter of the transistor Q104 is inputted to a base of the transistor Q101.
The amplifier 100 changes the switchover voltage Vmod so as to change a base bias, thereby controlling operation of the transistor Q101. Specifically, when high power output operation is performed, the amplifier 100 sets the switchover voltage Vmod as 3V so as to allow the bias circuit 102 to supply a base bias to the transistor Q101. On the other hand, when low power output operation is performed, the amplifier 100 sets the switchover voltage Vmod as 0V so as not to allow the bias circuit 102 to supply abase bias to the transistor Q101. The base bias of the transistor Q101 is the reference voltage Vref which is supplied via the resistance R101. The circuit configuration as described above allows the amplifier 100 to perform the changeover operation.
However, the conventional radio frequency amplifier circuit as described above has the following problems.
The first problem is that it is necessary to provide a high precision power supply for the power supply voltage Vdc and a high precision power supply for the reference voltage Vref.
The reason for providing the high precision power supplies is as follows. When the amplifier 100 performs high power output and a value of the reference voltage Vref is changed, a current flowing from the transistor Q104 corresponding to a base current for the transistor Q101 is changed, thereby significantly changing an operating current of the transistor Q101. On the other hand, when the amplifier 100 performs low power output and a value of the power supply voltage Vdc is changed, a base current for the transistor Q101 is changed, thereby significantly changing an operating current of the transistor Q101. Further, it is necessary to provide power supplies regulated by a voltage circuit for the switchover voltage Vmod and the power supply voltage Vdc, thereby increasing a circuit scale.
The second problem is that the amplifier 100 capable of changing between the high power output operation and the low power output operation requires an increased circuit scale.
The reason for increasing the circuit scale is as follows. When a control logic of the mobile communication terminal for changing between the high power output operation and the low power output operation is different from that of the amplifier 100 (the high power output operation: reference voltage Vref=2.7V, and the low power output operation: reference voltage Vref=0V), it is necessary to add, to a control circuit, a logic circuit for reversing a control signal, thereby increasing the circuit scale.
The third problem is that a power gain of the radio frequency amplifier circuit is different between when the amplifier 100 performs the high power output operation and when the amplifier 100 performs the low power output operation.
The reason the power gain is changed is as follows. A current flowing through the collector of the transistor Q101 is different between the high power output operation and the low power output operation. In general, in the radio frequency amplifier circuit using a transistor, increase in operating current leads to increase in power gain. Therefore, the power gain changes for each operation, and therefore a control parameter is required to have the increased number of values in the radio frequency circuit block of the mobile communication terminal, thereby complicating the control circuit.